Process for the electrolytic formation of aluminum coatings on metallic surfaces in molten salt bath



United States Patent 3,480,521 PROCESS FOR THE ELECTROLYTIC FORMATION OF ALUMINUM COATINGS ON METALLIC SUR- FACES IN MOLTEN SALT BATH Akira Miyata, Chikayoshi Tomita, and Akio Suzuki, Tokyo, and Hideyo Okubo and Masahiko Nagakuni, Kanagawa-ken, Japan, assignors to Nippon Kokan Kabushiki Kaisha, Chiyoda-ku, Tokyo, Japan No Drawing. Filed Nov. 9, 1965, Ser. No. 507,034 Claims priority, application Japan, Nov. 13, 1964, 39/63,827; Feb. 24, 1965, 40/10,330, 40/10,331 Int. Cl. C22d 3/12; C23b 5/22 U.S. Cl. 204-39 1 Claim ABSTRACT OF THE DISCLOSURE Process for coating electrolytically metallic surfaces with aluminum in a molten salt bath having an aluminum anode and containing aluminum chloride, which comprises the improvement of intentionally supplying hydrogen ions to the reacting zone between the metallic surface and the molten salts by positively supplying, in the form of small bubbles, gaseous hydrogen or hydrogen chloride while the electrolytic coating process proceeds, said hydrogen or hydrogen chloride being present in the bath in ionized state. The process results in coatings of superior smoothness and bonding strength between the surfaces.

SUMMARY OF THE INVENTION This invention relates to a process for coating steel, copper and other metallic surfaces with aluminum as protecting films by means of molten salt electrolysis in a fused bath containing aluminum halide and the like.

It is commonly known to those skilled in the art that various and considerable drawbacks are encountered for carrying out this kind of electrolytic process. Among others, a speedy and unavoidable deterioration of the electrolyte was encountered in the course of repeated electrolysis. This kind of deterioration will invite overall or localized loose deposit of aluminum coating on the metallic surface to be protected, prevent favorable higher cathodic current density from being adopted or result even in a formation of trees or slug-like non-tight deposits on the metallic surface. In extreme cases, the coating will represent powder-like appearance and is highly liable to be scaled-off even in the course of the water cleaning stage which is conventionally carried out in direct succession to the electrolytic coating process.

For avoiding these drawbacks, addition of various salts such as those of lead, cadmium, chromium and/or the like, has hitherto been proposed and employed. Inclusion of lead and other metals, up to 0.4%, in the aluminum coating, resulted however in a lowered anticorrosive performance, an accelerated scale-off tendency, and a rough touch of the coated aluminum film.

It is therefore the main object of the invention to provide an improved process for electrolytically coating steel and other metallic surfaces with aluminum in a fused salt chloride bath, providing and assuring a substantially extended durable life of the bathadapted for the electrolysis.

Another object of the invention is to provide a process of the above kind, capable of obtaining aluminum-coated metallic products having superior bond of the coating to the metallic stock electrolytically treated.

It is still another object to provide a process of the kind above referred to, capable of obtaining aluminumcoated metallic products having superior surface conditions of the coating thus produced.

It is a further object to provide a molten salt electrolytic process easy to operate at a high efiiciency.

These and further objects, features and advantages of the invention will appear more specifically and clearly as the descirption proceeds.

In the improved process as proposed by the present invention, hydrogen ions are supplied in either or both positive and negative sense while the process proceeds. There are numerous ways for carrying out the process. As a representative way to positively supply the hydrogen ions, electrochemically ionized hydrogen, for instance, by contact with platinum black, and/ or hydrogen chloride gas may be fed directly to the electrolytic bath.

Superior results obtainable by the inventive process can be attributable to the fact that the aluminum as deposited is loaded with hydrogen ion in a direct or an indirect manner.

An additional or alternative way for the supply of hydrogen ions is to provide prior to the electrolysis a very thin aqueous film on the metallic surface to be aluminumcoated, and only to subject the stock to the coating electrolysis afterwards. The precise mechanism, why hydrogen ions can be supplied by adopting such measure, is not clear to us at the present moment. It can be assumed with a certain degree of probability that the aqueous content of the preliminarily formed and maintained film will react \g/ith AlCl contained in the molten salt bath as shown elow:

In this way, hydrogen ions may be supplied to the electrolytically reacting zone in the course of the coating process. As the wetting solution, aqueous acetone solution of any desired concentration may preferably be used. According to the our experiments, superior results may be obtained when the aqueous wetting solution contains, solely or in combination, inorganic and/or organic substances or chemicals which have a high afiinity to water. For this purpose, methyl and/or ethyl alcohol, ethylene glycol, glycerine, lactic acid and/or formic acid may be employed.

Starch, sugar, casein, egg albumen, gelatin, water glass or the like organic and inorganic substances may be utilized for the same purpose, in the form of syrup or paste.

Various inorganic compounds such as sodium chloride, magnesium chloride, potassium chloride, calcium sulfate, magnesium perchlorate, caustic soda, calcium chloride, aluminum chloride, barium chloride, zinc chloride, chromium chloride, titanium tetrachloride, and the like may also be used in the form of an aqueous or alcoholic solution for the same purpose. When necessary, conventional surface active agents, for instance, soap, various known organic derivatives, such as sodium salts of .high molecular alkyl sulfates, or sulphonates may also be used. These agents may be used in the form of a dilute aqueous or alcohol solution for the purpose of this invention.

Even when these substances are dried up, they will contain a slight amount of water, thus in efiect, in the form of an aqueous film which can be utilized, according to this invention, in the aforementioned manner.

In the art of molten salt electrolysis for the formation of aluminum coatings, an aluminum mass is frequently submerged in the fused bath so as to act as anodic electrode, and to replenish the consumed quantity of the aluminum component in the bath as the electrolytic process proceeds-The mass dissolves out gradually in the molten hot bath regardless of conducting the electrolytic current and in excess of the consumed quantity of aluminum in the both. This superfluous solute .is liable to be reduced to metallic aluminum which is highly unstable and will act adversely on the desired electrolytic formation of aluminum coating, as will be specifically described hereinafter by way of an experiment set forth in Example 3.

As the unstable metal aluminum appears in the bath in the above-mentioned manner is oxidized to its ionized state in the presence of hydrogen ions as supplied according to the main feature of the invention, as in such a case metallic aluminum is liable to be oxidized in the presence of hydrogen ions when hydrochloric acid is supplied for contact therewith. In this way, the aforementioned drawback adversely affecting upon the ideal electrolytic formation of aluminum coating may effectively be obviated relying upon the novel teaching of the present invention.

In the inventive process, the supply of hydrogen ions to the electrolytically reacting zone can be carried out in negative sense in the following way:

More specifically, the metal stock is preparatorily and cathodically subjected to an electrolytic treatment in the presence of an acid solution containing cations of those metals which have higher hydrogen overvoltage than that for aluminum, such as Pb, Sn, Ti, Zn, Bi, Cr, Cd and/ or the like. By this preparatory treatment, these metals are deposited on the stock and carried by the stock to the electrolytically reacting zone in the next succeeding electrolytic aluminum coating process. When the latter process is carried out in the presence of these metals, hydrogen ions contained in the fused salt bath, especially those existing in close proximity of the electrolytically reacting zone, more specifically in the neighborhood of the metallic surface to be subjected to the aluminum coating process, are retarded in the transformation into hydrogen molecules. Although this retarding period is short, it is enough to carry out the electrolytic aluminum coating process under favorable conditions. In this way, hydrogen ions may be supplied in negative sense to the reacting zone, without relying upon any supply source outside of the fused salt bath.

Example 1 Using a molten bath, 200 00., comprising 60 mole percent of aluminum chloride and 40 mole percent of sodium chloride, a steel stock having the size of x x 0.2 mm, was in a conventional manner electrolytically treated as cathode, below which a length, say cm., of coiled platinum wire of 0.5 mm. thickness and 10 mm. coil diameter was held as anode under tension at a distance of 2 cm. while being submerged in the bath. The cathodic current density was 2 amp/din. and the electrolysis was continued for 10 minutes at 160 C. In the course of the electrolysis, hydrogen gas was supplied in the form of small bubbles at a rate of cc./min. to the bath from below so as to contact the coiled anode. In this way, an aluminum coating having a thickness of 3 microns and superior surface conditions and an excellent bonding strength could be formed on both surfaces of the sheet stock.

Example 2 For comparison, argon gas, again in the form of small bubbles, was introduced instead of hydrogen under the same operating conditions and at the same feeding rate as before and throughout the whole electrolytic period, which was again 10 minutes. An aluminum coating was produced on the steelstock, with no appreciable improvements in the quality of the coating. Though agitation of the bath was utilized, no appreciable gain in the desired results were achieved, because of the lack of the hydrogen ion.

Example 3 1180 gms. of aluminum chloride and 320 gms. of sodium chloride were thoroughly mixed together, heated to melting temperature, charged into a glass vessel and kept at a temperature of 160 C., whereupon the vessel was sealingly closed off from the ambient atmosphere. Before the vessel was sealed, a sheet of aluminum plate, having the dimensions of x 150 x 0.2 mm., was arranged in the melt as anode and a piece of steel sheet having the same dimensions as cathode; current was con- Wt. of Current Al-coil, efiieiency, Time lapsed gms. State of Al-coatmg percent None 44. 9408 Smooth, tight and Well- 83 bonded. 24 hours... 41. 5588 Same as above 69 hours 40. 4352 Formatlon of trees, 68

partially. 141 hours 38. 9280 Formation of trees, overalL. 40 261 hours 37. 1280 Pulverized state, overall. 15

Even with only a slight formation of trees, the deposited aluminum was liable to be scaled off in the course of the succeeding water cleaning step, thus the apparent current efliciency became actually lower and in a suddenly decreasing manner. When the anodic current efiiciency was measured relative to the dissolved-out quantity of aluminum, it amounted frequently to more than thus the aluminum content of the bath increased with the time duration of the electrolytic treatment. It wasobserved that aluminum once deposited on the cathode dissolved out again, as evidenced by a reduction of weight of the aluminum wire.

On the other hand, likewise in the above experiment, when the aluminum wire coil was kept in its submerged state in the molten bath at 160 C. for 30 days, a further dissolving-out of the aluminum could not be observed. When the bath temperature was reduced to C., air bubbles entering the bath were observed to be accompanied on the marginal surface thereof by separated aluminum in the form of thin scales in crystalline state, which is also a sign of the presence of the said superfluous solute of aluminum.

Example 4' Using a molten bath comprising 62 mole percent aluminum chloride and 38 mole percent sodium chloride, but without any addition of conventional metal mist suppressing agent such as potassium chloride so as to clearly observe the aforementioned adverse effects by the presence of superfluous aluminum solute as set forth in the foregoing Example 3, the electrolytic treatment was carried out at C. for about 10 minutes with a cathodic current density: 2 amp./dm. taking an aluminum plate as anode and a steel sheet as cathode, having the same dimensions as before, respectively, the voltage being 0.3- 0.4 volt and the treatment being repeatedly performed from several times to about 20 times per day and for an extended time period, such as 10 months. On the first day, the current efiiciency amounted to 85-90% which decreased to 62-70% on the third day. On the tenth day, the efficiency decreased to as low as 1020%.This made it very difficult to carry out theelectrolytic coating process with satisfying quality of the producedcoating.

When, however, gaseous hydrogen chloride was supplied to the molten bath on the third day at a rate of 80 cc./min. per one lit. of the bath material and for about an hour, so as to provide hydrogen ions to the electrolytically reacting zone, the current efiiciency was restored to 80% from the preceding value of 62-70%. When the duration of the supply of gaseous hydrogen chloride was further extended for 1.5 hours, the efiiciency rose to 85% which high value could be maintained for as long as six months by adopting the blowing-inoperation of hydrogen chloride while continuing the electrolytic coating operation. I

Example 5 A steel sheet stock, having the same dimensions as set forth in Example 3, was degreased, pickled and Well water-cleaned, and then was treated as cathode in an aqueous solution containing 1% of HCl and 0.005% of PbCl at a-current density of l amp./dm. for about 10 seconds, using a carbon electrode of equal dimensions as above as anode. The stock was then water-cleaned to a satisfying degree. In this case, the stock should preferably be subjected to the influence of mechanical vibration, preferably at several to about 100 cycles per second.

By the water-cleaning, soluble salts adhering to the steel surface were substantially removed therefrom, and then the stock was dried.

Should the treating liquid contain a higher concentration of PbCl than the above-specified value of 0.005%, the treated steel surface represented, to a slight degree, a brown to gray coloring; but still exhibiting the natural glazing color of steel.

On the other hand, when the PbCI -content was lower than 0.00l%, the appearance of the treated steel surface was substantially unchanged.

The thus treated stock was then subjected to a conventional electrolytic treatment in a molten salt bath comprising 5 8 mole percent of AlCl and 42 mole percent of NaCl according to the conventional technique for the formation of aluminum coating, providing thus a smooth, glazing and well-bonded aluminum film on the stock and capable of being subjected, without any cracks in the coating, to a subsequent mechanical working for the fabrication of finished products. The thickness of the aluminum could be adjusted with ease to a value thicker than microns which constitutes a considerable progress in the art.

Example 6 In this experiment, the content of PbCl was adjusted to 0.005% and the concentration of HCl was 3 N, the current density being 1 amp./dm. The preliminary treatment was carried out for about 30 seconds, further conditions for the preliminary step being same as described in Example 5.

The pretreated stock was further treated electrolytically with a current density of 2 amp./dm. as in the preceding Example 5, thus providing an aluminum coating, 17.4,u thick, with a current efliciency of 88%. The coating was beautiful, of metal glazing tone, and well-bonded with superior quality.

Example 7 In this experiment, the content of PbCl was adjusted to 0.0006% and the concentration of HCl was 0.3 N, the current density being 1 amp./dm. The preliminary treatment was carried out for about 30 seconds, further conditions for the preliminary step being same as described in Example 5.

The thus treated test piece was further treated electrolytically with a current density of l amp./dm. in a bath which had become highly deteriorated so that only a degreased and pickled stock could not have been treated therewith effectively, other conditions being similar to those as employed in Example 5. The thus formed aluminum coating was beautiful in its metal glazing tone, and well-bonded with superior quality.

Example 8 In this case, the aqueous preliminarily treating bath contained 0.1% of bismuth oxide and 1.0% of perchloric acid. The treatment was carried out with a current density of 1 amp./dm. for about 30 seconds.

Then, the treated stock was subjected further to an electrolytic treatment while using the same molten salt bath as was set forth in Example 7, and using a current density of l amp./dm. Superior results in the formed aluminum coating were obtained. In this case, however, the coating thus produced should be exposed to the open atmosphere at least once in the course of the next succeeding water-cleaning step, so as to convert the produced coating into an inactivated one.

Example 9 In this case, an aqueous solution of hydrochloric acid having a concentration of l N and containing 0.05% of stannous chloride was used as preliminarily treating liquor. The treatment was carried out with a current density of 1 amp./dm. for about 30 seconds.

Then, the treated stock was subjected further to an electrolytic treatment while using the same molten bath as was set forth in Example 7, and with a current density of 1 amp./dm. again. Aluminum film was coated on the stock with superior results.

Example 10 In this case, the preliminarily treating aqueous liquor contained 0.5% of lead acetate. The treatment was carried out with a current density of 0.3 amp./dm. for about 2 minutes.

The thus treated stock was then subjected to the second treatment, as before, for coating it with aluminum, yet while using same bath composition as set forth in Example 6. The coating was slightly inferior in its characteristics, yet having a superior metallic glaze.

Example 11 The preliminary treatment was carried out with use of an aqueous 2 N hydrochloric acid solution, containing 0.5 of titanium chloride at a current density of 20 amp./dm. t

Then, the thus treated stock was further subjected to an electrolytic coating treatment while using the same molten salt bath as set forth in Example 7. The current density was 1 amp./dm. as before. The thus obtained aluminum coating had a superior metal glaze, providing in addition, superior tightness, smoothness and bond.

Example 12 A steel sheet stock was preliminarily coated with a thin film of acetone, of purity, by the way of dipping.

Then, the treated stock was subjected to an ordinary electrolytic treatment utilizing a used up molten bath comprising 58 mole percent of AlCl and 42 mole percent of NaCl and kept at C., which bath had so deteriorated as to be considered unusable for conventional operations. The electrolysis was carried out with a current density of 1.5 amp./dm. for about 10 minutes. However, the thus resulting aluminum coating was highly smooth, tight, closely adhering and highly durable to scratching.

The acetone was replaced in subsequent tests by methyl alcohol, ethyl alcohol of 96% purity; ethylene glycol, 95 glycerin, 95 lactic acid, 87%; and formic acid, 87%; the effects were similar.

Example 13 An aqueous syrup (concentration: 1%) made of starch was used in place of the acetone used in Example 12, and the treating conditions were the same as set forth in that example. Similar superior results were obtained.

When the starch was replaced in succession by sugar (concentration: 10% casein (2% egg albumen (1% gelatin (1%) and Water glass (0.5%), similar superior results were obtained.

Example 14 A dilute aqueous solution of common salt (concentration: 5%) was used and other treating conditions were the same as set forth in Example 13. Same superior aluminum coating was obtained.

When the common salt was replaced in succession by magnesium chloride (5%), potassium chloride (5 calcium sulfate (0.2%), magnesium perchlorate (0.5 caustic soda (1%), calcium chloride (5%), aluminum 7 chloride barium chloride (2%), Zinc chloride (2%) and chromium chloride (1%), substantially the same superior results were obtained.

The dipped metal stock was dried at normal temperature and then left in the open air so as to absorb a certain quantity of water content from the ambient air, so as to form a thin aqueous film on the stock surface.

When any one of the above chemicals was replaced by a small amount of conventional surface active agents such as alkylated sodium benzenesulfonate', or added in addition, similar results were obtained.

Example A combination of two or more of the substances referred to in the foregoing Examples 12-14 could be employed with equal results when other operating conditions Were similar to those set forth therein.

' For instance, ethyl alcohol or an aqueous solution thereof was added with a small amount of TiCl and coated in a thin film on a steel or other metallic sheet stock and the latter was treated under similar conditions as before, the resulting aluminum coating was highly superior due to the combined action of both chemicals. In this case, the stock after being dipped, was treated while in its wet condition. Even when the preliminarily treated stock was dried in a hot air drier and then treated as before, the nature and conditions of the obtained aluminum coating were substantially the same as before.

Although steel stocks have only been described hereinbefore, other various metallic stocks such as copper and the like may equally be treated by the inventive process. This will apply also to other metals than steel.

What is claimed is:

1. In the process for electrolytically coating a metallic surface with aluminum in a molten salt bath containing aluminum chloride and wherein an aluminum metal member is used as anode and the metallic surface to be coated as cathode; the improvement wherein, as the electrolytic process proceeds, hydrogen ions are supplied into the bath from an external source in the form of small bubbles for interaction with the aluminum ions to improve the coating on the metallic surface and wherein the source of hydrogen ions consists of a member selected from the group consisting of hydrogen and hydrogen chloride.

References Cited UNITED STATES PATENTS 3,288,689 11/1966 Azuma 204-39 XR 527,846 10/1894 Waldo et al. 204-67 527,847 10/1894 Gooch et al. 204-67 527,848 10/1894 Gooch et al. 20467 528,365 10/1894 Gooch et al. 204246 XR 2,752,303 6/1956 Cooper 204246 XR 2,900,319 8/1959 Ferrand 204246 XR 3,007,854 11/1961 Smith et al. 20439 XR 3,136,709 6/1964 Smith et al. 20439 XR JOHN H. MACK, Primary Examiner G. L. KAPLAN, Assistant Examiner US. Cl. X.R. 20467 

